1. Field of the Invention
The present invention relates to gas burner control and more particularly to feedback control for modulating gas burners.
2. Description of the Prior Art
Gas burners employ a source of gas which passes through a regulator to control the flow emitted through an orifice. A source of air is mixed with the gas and the gas/air mixture is transmitted to a burner where an igniter causes combustion. The resulting flames are thrown past a flame sensor into a heat exchanger that transfers heat to a supply of air directed to the space to be heated. The flow of burning gas/air mixture in the heat exchanger is controlled by a combustion fan at one end. The gas/air flow is proportional to the RPM of the fan which is typically supervised by an air pressure switch. Changes in fan speed cause changes in the amount of heat exchanged and the heat that is directed to the space to be heated may be controlled. However, as the speed of the fan is changed, the ratio of gas to air in the gas/air mixture must also be changed to maintain good combustion and keep efficiency within an acceptable range
It is known that the ratio of gas to air in the gas/air mixture needs to be within certain limits in order to provide good combustion and efficiency. The gas flow may be controlled by an electric modulating gas valve with a gas pressure regulator. Modulating gas burners have been constructed to attempt to obtain the desired gas/air mixture under various conditions but existing modulating gas burners normally rely on open loop control of the gas and air relationship. This leads to two problems: the first is the production tolerance of the modulating gas valve and the second is the tolerance of the combustion air control system.
In FIG. 1, the modulating signal to two hypothetical production valves is shown plotted against the percent of maximum output pressure. The variation of a high limit valve in a typical production batch may be shown by line 2 and the variation of a low limit valve from the batch may be shown by line 4. The values for the modulating signals are arbitrary values representing desired output pressures. For example, if an output is desired to be 40% of the maximum, the modulating signal request may be set at 40. However, because of the variations in the valves of a batch, it is seen that the valves representing the high and low members of the batch may produce outputs between about 30% and about 43% of the maximum when the modulating signal is set at an output request of level 40. For optimal efficiency, this range should be lower and while the range can be lowered by achieving tighter tolerances for the modulating valves in a batch, this is quite impractical for a low cost gas valve.